An acoustic charge transport device ("ACT device") is a novel analog semiconductor device which is particularly useful as a delay line and for performing various signal processing functions. An ACT device overcomes many of the problems of the prior charge-coupled devices ("CCD") because the ACT device processes signals at extremely high speeds while avoiding the necessity of clock driver and certain other circuitry. An ACT device achieves its high processing speed because a surface acoustic wave ("SAW") propagates through a channel buried within the piezoelectric semiconductor substrate and the wave transports the charge through the channel. The charge is transported in discrete identifiable packets which move at the speed of the wave.
Transportation of charge through the buried channel produces an output at the drain (output contact) which is in the form of sharp current pulses. These current pulses originate from the drain only when the SAW voltage under the drain is positive, because the electron packets are carried in the positive half cycles of the SAW voltage. Therefore, it is possible to monitor the pulses after transport through the channel and to determine the quantity of charge that was transported.
The voltage-to-charge conversion process occurring at the ACT device source (input contact) is one wherein the amount of charge injected into each packet is proportional to the input voltage at specific sample points in time. In the ACT device, these sample points tend to occur at the instants in time when the positive peaks of the SAW potential are under the source. Hence, the input section of the ACT device operates as an automatic time domain sampler having a sampling rate equal to the SAW frequency.
In theory, an ideal sampler would produce a sample (charge packet) which is proportional to the analog input voltage at specific exact instants in time. The achievement of the ideal sampler, however, requires a physical process or circuit element which can turn on and off, or switch, infinitely fast. Actual physical devices have finite switching times which cause the sample amplitude to be proportional to a weighted average of the input signal over the switching time. This switching time is referred to as the sampling aperture. The primary impact of a finite sampling aperture is to limit the precision with which the sample can be formed. This, in turn, tends to limit the highest frequency of input signal which can be unambiguously sampled. The sampling aperture of the input sampling mechanism of the ACT device is very short, and is substantially equal to approximately one tenth (1/10) of the SAW period. Hence, in principle, the ACT device input is capable of high speed voltage-to-charge conversion over a wide bandwidth of input frequencies, up to approximately ten times the SAW frequency.
Prior art structures have not been able to take full advantage of the high speed input properties of the ACT device. The prior structures limited the sampling rate to the SAW frequency, even though the input charge injection process is capable of injecting charge at a rate substantially equal to approximately ten times the SAW frequency. In analog signal processing applications, the limitation of the sampling rate at the SAW frequency also limits the signal bandwidth obtained in the devices to one half of the SAW frequency, because of the Nyquist criteria.
In view of the above, it can be seen that there is a need for a structure permitting maximum utilization of the high speed processing capabilities of the ACT device. The disclosed invention is a novel arrangement of at least two ACT devices, so that the waves are complementary at specific points, in a manner which overcomes the limitation of the SAW frequency sample rate. In addition, the Nyquist limited signal bandwidth is correspondingly extended in certain embodiments.
The complementary ACT device ("C-ACT device") is a circuit element which combines two or more similar ACT devices by the parallel connection of the sources and/or the drains thereof. The SAWs of the devices are complementary in phase with respect to the connected sources and/or drains, so that the input sampling time intervals and/or the output detection intervals are staggered, thereby permitting input sampling and output detection at a rate equal to N times the SAW frequency, where N is the number of devices which are connected. The invention may be realized by externally interconnecting discrete devices, or by integrating the required multiple ACT transport channels on one chip in a parallel configuration. In the latter case, which is particularly useful for achieving the required accurate signal balance between channels, the composite circuit element forms a new device with terminal properties considerably different from those obtained from a prior art ACT device.